Exploring the local solvation structure of redox molecules in a mixed solvent for increasing the Seebeck coefficient of thermocells

Abstract

A thermocell is an emerging alternative to thermoelectric devices and exhibits a high Seebeck coefficient (S_e) due to the large change of solvation entropy associated with redox reactions. Here, the S_e of p-chloranil radicals/dianions (CA˙^−/2−) in acetonitrile was drastically increased from −1.3 to −2.6 mV K⁻¹ by the addition of ethanol, and the increment surpassed the estimation of the classical Born model with continuum solvent media. UV-vis spectroscopy and electrochemical measurements at various mixing ratios of acetonitrile to ethanol revealed that the strong hydrogen bonding between ethanol and oxygen atoms of CA²⁻ forms a 4 : 1 solvent–ion pair, while the ethanol molecules binding to CA²⁻ dissociate upon its oxidation to CA˙⁻. The local solvation structures of CA²⁻ are in good agreement with density functional theory. This order–disorder transition of the local solvation structure around the CA˙^−/2− ions produces a large entropy change and results in a large S_e value. The tailored solvation structure of redox ions by hydrogen bonding is a versatile method applicable to a variety of redox pairs and solvents, contributing to the development of electrolyte engineering for thermocells.